A number of chemical analyses require the sample to be tested in a fine homogeneous powdered form. This is the case with some tests performed on ore samples, which require the sample to be pulverised into particles of diameters smaller than 75 microns.
Some analyses also require large quantities of the sample to be sampled. The analysis in question may consume a large amount of the sample, or several analyses may be required to ensure the results obtained are repeatable.
In the case of analysing the chemical components of ore in the form of rocks, the preparation of the sample proposes several problems. Large quantities of rock need to be pulverised into a very fine powder for some giving a homogeneous powdered sample.
An existing method of pulverising rock samples is to use a ring mill.
A ring mill consists of a bowl within which is placed one or more solid rings and a centrally located solid puck. A lid is also provided which can clamp down solidly on the bowl. The bowl is fixed to a horizontal platform mounted on a set of springs. A driving motor which vibrates the bowl is attached to the underside of the platform.
When a rock sample is placed inside this mill the larger particles are crushed between the outer ring and the bowl walls, between the rings, and between the puck and adjacent rings. Crushing also occurs between the upper and lower surfaces of the ring, the bowl and lid.
The ring mill is driven until the sample within consists of a homogeneous mix of particles of the right size.
This mill design is effective but has a major problem which disadvantages the operator.
A ring mill can only process small amounts of sample in one processing operation. If too much sample is added to the bowl it ends up choking up the puck and rings, limiting the movement of the rings when the device is driven. This causes a severe problem, as pulverisation of enough sample for an analysis turns into a long and tedious job, with several processing stages being required.
In some instances crushing of the sample is required before it is fed into the ring mill. If the materials are too large or coarse the ring mill can not effectively pulverise the sample, or will require an extremely long pulverising period within which to process a sample to their required particle size and homogeneity.
Another type of pulverising mill is the discus mill described in Australian Patent No. 570814.
The discus mill uses the same driving mechanism as a standard ring mill, but replaces the ring and central puck with a discus which has a convex curved base. The discus also includes a cone shaped aperture located off centre to the middle of the discus. This mill employs a concave shaped base to its bowl, in which the discus moves.
When vibrated the discus is able to run up the walls of the bowl, while trapping and pulverising particles of the sample underneath when falling down from the side walls of the bowl. The aperture in the discus acts to distribute material under the discus.
The discus mill solves some of the disadvantages involved with using a ring mill because it can process a large amount of sample. However, the discus mill still relies on long processing times to pulverise a sample down to the required sized particles.
This results in slow sample preparation. An operator is again limited in the amount of sample they can process in a particular period of time.
An additional problem associated with discus mills is removal of the sample once pulverisation has occurred. The discs employed in a conventional discus mill can weigh in excess of 25 kilos, which operators find difficult to lift and move easily. Consequently a hydraulic or pneumatic lifting device is required to remove the discus from the mill.
This results in increased expense, as more equipment is required for sample preparation, and also slows down sample preparation, the operator has to control and manoeuvre a secondary piece of mechanical apparatus to allow the sample to be removed from the Mill.
When used extensively the discus from a discus mill is reduced in weight by the abrasion of the sample it grinds. The performance of the discus will fall steadily with use as its weight decreases, until it must be replaced because pulverising times are too long.
In a single discus mill the base of the bowl used slowly wears away until the bowl needs replacing. The bowl is an expensive component, and because in single discus mills a curved bowl base is used, the bowl cannot easily be refurbished with a replacement base plate.
A different type of pulverising apparatus, termed a ball mill may be used in continuous flow processing applications. Flow processing-mill apparatus is used in a production line assembly where material is continually added to the milling apparatus in a coarse form and removed from the apparatus in a fine ground form. This may be contrasted with batch processing operations where a milling device contains a set amount of sample and is operated for a measured period of time, stopped and then the sample removed.
Ball mills used in continuous flow processing applications may be configured in a number of ways. However, all ball mills include a main receptacle to which is added a number of balls, along with the material to be ground. The receptacle is then rotated, or generally agitated to move the balls against the material to be ground.
As can be appreciated by one skilled in the art ball mills do not operate as efficiently as other milling and pulverising devices. The ball will have only a point contact with another ball and the material to be pulverised in between the two balls. Only a small amount of material will be pulverised with each impact of a ball, due to the small surface area contact that occurs. This make ball mills relatively inefficient when compared to other forms of milling apparatus.
A milling and pulverising device which overcame the problems listed above would be a great advantage over the existing prior art. Such a machine would greatly increase the speed of sample preparation by reducing the time periods involved in preparing a batch of sample and by processing more sample in one batch than normally possible.
Some laboratories process thousands of samples a day and therefore any reduction in pulverising time is a considerable cost and labour saving. In addition, a pulverising and milling device which accepted coarse material would again speed up the sample process, eliminating the need for crushing of a sample.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.